This invention relates generally to a process for forming an alloy layer on the surface of a substrate.
It is customary in the art of electrophotography to form an electrostatic latent image on a photoreceptor drum or plate comprising an electrically conductive backing such as, for example, a metallic or metal-coated base having an inorganic photoconductive insulating layer applied thereto in good charge blocking contact. Typical plates or drums comprise, for example, an aluminum surface having a thin layer of vitreous selenium with an aluminum oxide and/or polymeric interlayer. Such elements are characterized by being capable of accepting and retaining a suitable uniform electrostatic charge in the dark and of quickly and selectively dissipating a substantial part of the charge when exposed to a light pattern.
Electrophotographic elements have been modified in recent years to improve various properties including range of spectral response, heat and charge stability, greater discharge rates, and the like. These improvements may be achieved, for example, by the addition of various alloying components, or other types of additives such as those described in U.S. Pat. Nos. 2,803,542, 2,822,300 and 4,015,029. The addition of various amounts of a vaporizable alloying component, such as tellurium or arsenic, can result in a broad range of changes in the sensitivity, photographic speed, photographic stability and/or other properties of an electrophotographic imaging member.
Suitable alloys or homogeneous mixtures of elemental selenium with other metals suitable for alloying can be admixed and applied by conventional vacuum evaporation techniques. For example, inorganic coating materials may be placed in open or shuttered crucibles during an initial coating step. The xerographic substrate upon which the photoconductive material is to be deposited may, for example, be placed above or in some other suitable location with respect to the coating vapor source. After the chamber has been evacuated to a suitable pressure, e.g. about 5.times.10.sup.-5 Torr, the vessel containing the photoconductive material and any additives is then generally heated by electrical resistance to effect vaporization of the material. At least some of the vaporized material then condenses on the relatively cool substrate. This type of deposition process normally requires a period of about 15 minutes to about 60 minutes, depending upon the surface area of substrate material to be coated and the desired thickness of the coating material.
It has been found desirable to control the concentration profile through the thickness of one or more photoconductive components in one or more separate layers of different photoconductive materials to obtain desirable photoreceptor characteristics and to avoid certain undesirable properties such as high dark discharge. In one technique, the respective photoconductive alloy components are applied to substrates by coevaporation techniques in which predetermined amounts of the respective photoconductive materials or alloys are placed in separate crucibles or in subdivided crucibles and exposed or heated in a predetermined sequence under vacuum. One possible modification for this purpose involves coating substrates in the presence of one or a plurality of elongated shuttered or unshuttered crucibles heated by electrical heating elements or by other conventional means, the crucibles being subdivided into a plurality of compartments or bins, each capable of carrying different premeasured amounts and kinds of coating materials depending upon the desired final concentration. Another possible modification involves the formation of one or more trains of smaller crucibles temporarily connected to each other and containing various photoconductive materials.
The foregoing modifications are very useful in coating a plurality of substrates simultaneously with a plurality of components. However, there are serious economic and technical limitations inherent in their use. For example, it is very difficult to maintain and efficiently operate mechanical devices such as crucible shutters for batch coating operations due to jamming caused by random condensation of photoconductive material within the vacuum coater. The alternative approach of employing weighed amounts of each desired component in a plurality of open, self-heating crucibles offers a partial solution to the problem except for the substantial expenditures of time and money required to fill a plurality of crucibles with different amounts of different alloying components during each batch coating operation. In addition, it is difficult to avoid contamination, to control spattering, and to control evaporation rate in a timed evaporation sequence due to uneven heat distribution or hot spots of a generally unpredictable nature within individual crucibles and their contents. Run-to-run reproducibility is an inherent problem in the above approaches.
The technical problems noted above can be partially minimized by the use of one or more open crucibles and a timed heating sequence, preferably with irradiation and heating devices such as infrared heat sources. Unfortunately, a number of inorganic photoconductive materials, including selenium and many useful alloys of selenium, are transparent or at least partially transparent to light of the longer wavelengths such as infrared. As a result, the crucible walls and bottom plus various hot spots within each crucible charge will heat up much faster than the upper surface of the crucible charge. This not only results in the inefficient use of energy input due to secondary radiation from the crucible walls and bottom, but may actually result in small explosions due to the build up of gases and cause serious spattering of the coating material with resulting defects on the surfaces being vacuum coated.
One technique for minimizing spattering is by incorporating nonvolatile infrared absorbing heating particles on or within the body of inorganic components prior to effecting evaporation of the components. However, because of the size of the spaces between any particles on the surface of the molten vaporizable alloying components, this technique does not significantly control the rate of deposition of a mixture containing at least one vaporizable alloying component having a higher vapor pressure than at least one other vaporizable alloying component. For example, it has been found that shotted alloys of selenium and tellurium generally fractionate appreciably during evaporation so that a coating having a thickness of about 60 micrometers can have a concentration of tellurium at the top surface of the coating of about 3 to 6 times the tellurium concentration of the starting mixture of selenium and tellurium. This high surface tellurium concentration can significantly reduce or even prevent charge retention on the photoreceptor. It is believed that because of the higher vapor pressure of the selenium it evaporates in greater concentrations early in the vacuum deposition process, thus resulting in progressively increasing concentration of the tellurium component within the container. Similar problems have also been encountered in preparing alloys of selenium and arsenic where the selenium component has a higher vapor pressure than the arsenic component. Various approaches have been taken to control the fractionation problem with limited success, particularly in attempts to reproduce equivalent results with each batch. For example, the alloy shot can be ground and formed into pellets and thereafter utilized in vacuum deposition processes. Although grinding and pelletizing reduces the tendency to form high concentrations of the lower vapor pressure component in the outer region of the deposited alloy layer, the concentration of the lower vapor pressure component at and near the surface of the deposited alloy layer is still undesireably high for many applications. Moreover the grinding and pelletizing steps require additional energy, equipment and time for processing, and may introduce some possible health hazards in the absence of adequate safeguards.
Thus, there is a continuing need for a better system for forming with minimal fractionation an alloy layer on the surface of a substrate where the alloy components comprise at least one vaporizable alloying component having a higher vapor pressure than at least one other vaporizable alloying component.